fix(gitfs): restore remote prune origin during fetch

lib/platform/git/storage.js

@@ -78,6 +78,7 @@ class Storage {
           await determineBaseBranch();
           await resetToBranch(config.baseBranch);
           await cleanLocalBranches();
+          await git.raw(['remote', 'prune', 'origin']);
           const fetchSeconds = Math.round(
             convertHrtime(process.hrtime(fetchStart)).seconds
           );